Magnetic and spectroscopic probes for FeOFe linkages in hemin systems

Magnetic and spectroscopic properties of mu-oxo-bis-hemins from natural and structurally related porphyrins were investigated as probes for ascertaining the presence or absence of FeIII-O-FeIII linkages between hemin moieties of hemeproteins. Magnetic susceptibilities of solids from 2.2 to 293 degrees K were investigated. The data fit the temperature variations expected for a pair of antiferromagnetically coupled S = 5/2, iron (III) porphyrins with J values of 175, 190, 195, 205, and 210 degrees K for deuterohemins with hydrogen, vinyl, 2'-ethoxycarbonylcyclopropyl, acetyl, propionyl, and ethyl 2,4-substituents, respectively. This magnetic character is reflected in PMR spectra that exhibit resonances with far less broadening and paramagnetic shift than is the case for monomeric high-spin hemins. Only impurities are seen in EPR spectra, which serve effectively in monitoring the magnetic purity of preparations. An infrared active asymmetric stretching frequency characteristic of the FeOFe linkage can be identified by substitution of 160 by 180. Electronic spectra are highly characteristic with poorly resolved absorption bands. The substituents on the porphyrin ring exert significant, but usually not large, electronic and steric effects on these properties. Solvent effects were relatively small and no firm evidence for binding of ligands trans to bridging oxygen was found. The uniqueness of these physical properties and their low sensitivity to changes in porphyrin structure or medium facilitates the identification of mu-oxo linkage in hemins or oxidized hemeproteins.